Heat-shrinkable films have widely been used for various industrial applications such as, for example, with shrink-wrap films, shrinkable labels and cap seals, by making use of their property of heat-dependent shrinkage. The films have been applied to a variety of surfaces, including plastic and glass surfaces. Shrink films have been manufactured from vinyl chloride resins, polystyrene resins or polyester resins; however, in many instances, film shrinkage is concomitant with undesirable “draw back” of the outer edges of the film (a.k.a. “smiling” or “frowning”). The undesirable draw back of the outer edges has limited the application of shrink films. Therefore, there is a need for shrink films that have little to no draw back.
As well, interest in compostable polymers, i.e. biopolymers, has greatly increased, and many companies have made efforts to market, for example, packaging materials, hygiene products, sacks, and films with compostable polymers. Polylactic acid (PLA), i.e., polylactide, or condensation polymers which are based on lactic acid, are for many reasons a very attractive group of biopolymers. Their principal degradation product, lactic acid, is a product common in nature, it is not toxic and is used widely in the food and pharmaceutical industries.
PLA films can be manufactured by either blown film or casting (e.g., cast and tenner) techniques. While either method has advantages and drawbacks, the cast film is generally better suited for certain end-user applications, such as those requiring film “sleeves” rather than “wrap-around” film. Further, cast film generally has much better optics than a blown film and can be produced at higher line speeds. However, PLA films that are manufactured by current casting methods exhibit excessive shrinkage in the machine direction, which substantially contributes to curling and limits their range of application. Thus, there is a need for PLA shrink films manufactured by casting methods that exhibit little to no shrinkage in the machine direction.